1 Field of the Invention
This invention relates to a process for manufacturing a stiffener made of composite material with an inside surface of a concave shape.
2 Description of the Related Art
In the field of aeronautics, stiffeners that are called stringers, connected to the inside surface of a wall of an aircraft fuselage, are known. On the geometric plane, this type of stiffener 10′ can have a U-shaped cross-section as illustrated in FIG. 1A. According to another aspect, these stiffeners can comprise discontinuities over their lengths, for example slopes on the order of 10% over several millimeters in particular because of other reinforcements that are present on the skin of the fuselage.
These U-shaped stiffeners can also be present at the level of other parts of the aircraft, such as the center wing section that connects the fuselage to the wing. The center section can comprise other types of stiffeners, as illustrated in FIG. 1B, at the junction zone of two adjacent panels that form said section.
According to one embodiment, this type of stiffener 10 comprises primary wings 12 and 12′ that form an angle of approximately 90° and secondary wings 14 and 14′, each connected to one of the primary wings 12 or 12′ and arranged approximately at 90° relative to this wing. These secondary wings 14 and 14′ have ends that are oriented toward one another although the distance that separates the ends of the secondary wings is less than that separating the edges 16, 16′ that connect the primary wings 12, 12′ to the secondary wings 14, 14′. The junction zones between the primary wings, on the one hand, and the primary wings and the secondary wings, on the other hand, can be more or less rounded.
In the case of a center section, the primary wings 12 and 12′ are flattened against the adjacent walls of the center section, and the secondary wings are flattened against the wings of the adjacent U-shaped stiffeners.
According to a first operating mode by press stamping, the stiffener is produced between a mold (or punch) and a counter-mold (or matrix).
In advance, layers and/or folds of fibers that are pre-impregnated with resin are draped flat, with the fibers being oriented correctly based in particular with respect to the stresses that will be applied to the stiffener. Next, strips are cut based on the dimensions of the stiffeners. For each stiffener, the corresponding strip is heated to a temperature on the order of 80° C. and then deposited between the mold and the counter-mold that are mounted on a heating press. The mold and the counter-mold are then closed in such a way as to form the stiffener. After a hot compacting phase whose purpose is to produce, on the one hand, an optimum degassing of the preform without drawing in the product that can form the matrix, and, on the other hand, a contraction of the preform, the mold and the counter-mold are open in such a way as to extract the stiffener that is formed.
This approach can make it possible to obtain partially polymerized stiffeners and to terminate the polymerization after the application of stiffeners on the skin to be reinforced in such a way as to obtain a connection by co-curing between said stiffeners and said skin.
This operating mode is not suitable for obtaining stiffeners with closed cross-sections, as illustrated in FIG. 1B, because in this case, the punch or the matrix is to be produced in several parts that move relative to one another to obtain this type of profile. The fact that the punch or the matrix is produced in several parts leads to complicating the piece of equipment, generally making it more fragile, and it does not ensure the absence of folding of the layers.
According to another operating mode by thermoforming, in a first step, a flat blank is produced by draping layers and/or pre-impregnated fibers of a resin, with the fibers being oriented correctly in particular with respect to the stresses that will be applied to the stiffener.
This flat blank is next placed on a punch-type piece of equipment and then heated to a temperature on the order of 80° C. in such a way as to make it deformable and to make possible the sliding of fiber layers relative to one another by preventing folding. To flatten the blank against the punch, a bladder is used that is connected in an airtight manner to the punch on the periphery of the blank to be deformed. Next, a partial vacuum is produced in the space that is delimited by the punch and the bladder in such a way as to flatten the bladder and therefore the blank against the punch.
The unit is next cooled to set the shape of the stiffener before the bladder is removed.
As above, this operating mode is not satisfactory for producing stiffeners with closed cross-sections, because it does not ensure a sliding of layers relative to one another over the entire established length of the primary and secondary wings although folds can appear, more particularly at junction zones of the wings.
Consequently, regardless of the operating mode, folds can be created during the shaping of the stiffener, whereby these folds constitute structural defects that alter the mechanical characteristics of the thus obtained stiffener.